1. Field of the Invention
The present invention relates to methods of designing sets of mask patterns for use in multiple-exposure lithography processes, to sets of mask patterns and to device manufacturing methods.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include “steppers” and “scanners”. In a stepper type lithographic apparatus each target portion is irradiated by exposing an entire pattern onto the target portion at one time. In a scanner type lithographic apparatus each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Known immersion lithographic techniques include a technique wherein there is an immersion of the substrate in a lithographic projection apparatus in a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the final element of the projection system and the substrate. If water is used, it is desirably distilled water. Other liquids can be used. The present invention will be described with reference to “liquid”. However, various fluids may be suitable, particularly wetting fluids, incompressible fluids and/or fluids with higher refractive index than air, desirably a higher refractive index than water. Fluids excluding gases are particularly preferred. An advantage derived from the use of fluids is to enable imaging of smaller features since exposure radiation will have a shorter wavelength in the liquid. The effect of the liquid may be regarded as increasing the effective numerical aperture (NA) of the system and increasing the depth of focus. Other immersion liquids have been utilized, including water with solid particles (e.g., quartz) suspended therein, or liquids with nano-particle suspensions (e.g., particles with a maximum dimension of up to about 10 nm). The suspended particles may or may not have a similar or the same refractive index as the liquid in which they are suspended. Other liquids, which may be suitable, are hydrocarbons, such as aromatics e.g., Decalin, and fluorohydrocarbons, and aqueous solutions.
However, submersing the substrate or substrate and substrate table in a bath of liquid, see, for example U.S. Pat. No. 4,509,852,which is incorporated by reference herein in its entirety, means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects.
A method for a liquid handling system, such as a liquid supply system, is to provide liquid on only a localized area of the substrate, such as, in between the final element of the projection system and the substrate using a liquid confinement structure (the substrate generally has a larger surface area than the final element of the projection system). One such method utilizing this approach is disclosed in WO 99/49504,which is incorporated by reference herein in its entirety. As illustrated in FIGS. 2 and 3 of this application, liquid is supplied by at least one inlet IN onto the substrate, desirably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side. FIG. 2 of this application shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of FIG. 2 the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of inlets and outlets positioned around the final element are possible. One example is illustrated in FIG. 3 in which four inlet/outlet sets are provided in a regular pattern around the final element.
Another method for a liquid handling system is to provide the system with a seal member which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table. Near the seal is a meniscus of the immersion liquid. The seal may confine the immersion liquid and so create the meniscus. As exposure light passes through the confined immersion liquid it may be considered optical liquid. Such an arrangement is illustrated in FIG. 4 of this application. The seal member is substantially stationary relative to the projection system in the XY plane though there may be some relative movement in the Z direction (in the direction of the optical axis). A seal is formed between the seal member and the surface of the substrate. Desirably, the seal is a contactless seal such as a gas seal, such as is disclosed in EP-A-1,420,298,which is incorporated by reference herein in its entirety and illustrated in FIG. 5 of this application.
EP-A-1,420,300,which is incorporated by reference herein in its entirety, discloses a twin or dual stage immersion lithography apparatus. Such an apparatus is provided with two stages for supporting the substrate. Leveling measurements are carried out with a stage at a first position, without immersion liquid, and exposure is carried out with a stage at a second position, where immersion liquid is present. Alternatively, the apparatus has only one stage.
WO2005/064405,which is incorporated by reference herein in its entirety, discloses an all wet arrangement in which the immersion liquid is unconfined. In such a system, the whole top surface of the substrate is covered in liquid. This is advantageous because then the whole top surface of the substrate is exposed to the same conditions. This has advantages for temperature control and processing of the substrate. In WO2005/064405,a liquid supply system provides liquid to the gap between the final element of the projection system and the substrate. The liquid between the final element of the projection system and the substrate during exposure is optical liquid. That liquid is allowed to leak over the remainder of the substrate. A barrier at the edge of a substrate table prevents the liquid from escaping so that it can be removed from the top surface of the substrate table in a controlled way. The meniscus of the liquid defining the extent of the immersion liquid is remote from the projection system. Although such a system improves temperature control and processing of the substrate, evaporation of the immersion liquid can still occur. One way of alleviating that problem is described in US 2006/119809,which is incorporated by reference herein in its entirety, in which a member is provided which covers the substrate W in all positions and which is arranged to have immersion liquid extending between it and the top surface of the substrate and/or substrate table which holds the substrate.
In order to produce features of ever smaller critical dimensions, various methods have been used to form features smaller than allowed by the diffraction limit. Some processes involve two lithographic steps to produce a single layer of a device. These methods are often referred to as double-patterning processes. One such process involves exposing a first resist layer provided over a hardmask with a first pattern, developing the first pattern and etching it into the hardmask, applying a second hardmask, a thick layer of anti-reflection coating (ARC) and a second resist layer, exposing the second resist layer with a second pattern, developing the second pattern and etching it into the second hardmask, then transferring the combined pattern in the hardmask into the substrate. Such a process, often referred to as a Litho-Etch-Litho-Etch or LELE process, is disadvantageous as the substrate must be removed from the lithocell (lithography apparatus and associated track) to perform the etch steps, which causes delay and provides additional opportunity for contamination of the substrate.
There have been various methods to “freeze” the first pattern in the first resist so that the second resist can be coated directly over it and exposed to the second pattern without eradicating the first pattern. This can be advantageous if the “freezing” process can be carried out without removing the substrate from the track. One suggested “freezing” process involves baking the developed first resist so that it undergoes a chemical reaction, e.g., cross-linking between polymer chains, rendering it less solvent in the solvent of the second resist. Another suggested “freezing” process involves coating the developed second resist with a substance that reacts with it, possibly when heated, to form a protective layer. Another suggested double patterning process avoids an intermediate etch step and uses a positive resist for the first exposure and a negative resist for the second exposure, ensuring that the features formed in the first exposure are not irradiated in the second exposure.
Other processes to produce smaller pitches than the limit of a single exposure step include so-called spacer techniques. In these processes, a resist is exposed with a first repeating pattern which is etched into a hardmask. The patterned hardmask is coated with a spacer material that forms a conformal coating. A controlled etch of the conformal coating is performed so that only spacers of controlled width on the sides of the hardmask features remain. The hardmask features are then etched away leaving only the spacers, which have a pitch half that of the hardmask features. There is also a dual-tone process which uses a resist that effectively has two thresholds to exposure radiation, which remains hard when exposed to the higher threshold and less than the lower threshold so that only areas that have been exposed to an intermediate amount are washed away in the development process.
However, none of the above processes is ideal. The double patterning processes involve two expensive lithography steps, even if an intermediate etch is avoided. They are also sensitive to overlay errors between the two patterning steps and other imaging errors. The spacer and dual-tone resist processes are typically only useful to form repeating structures, e.g., in memory, rather than logic-type structures. Therefore, what is needed is an improved process by which features of a critical dimension smaller than the limit of a single exposure step can be formed.